After last week's decision to shut down the mobile networks by the San Francisco's Bay Area Rapid Transit System -- BART, in short -- in a bit to disrupt a planned protest, hacktivist group Anonymous made good on its promise to cause its own disruption.

The hacktivist collective hacked the mybart.org website shortly after midday on Sunday, targeting it with a distributed denial of service (DDoS) attack -- with a twist.

As CNET reports, though the site was running slower than usual, one notable change was clear. The site had been defaced with the ominous symbol of Anonymous: the Guy Fawkes mask.

The bart.gov website, the BART system's main portal, remains online and unaffected, however.

Along with this, the mybart.org website had also been hacked into, and a vast database of user emails, addresses and phone numbers of website users has been leaked onto code-sharing website, Pastebin.

Other websites, including the California Office of Traffic Safety's 'California Avoid' site has also been defaced.

BART, in a statement, warned of potential disruption to its online services -- responding directly to Anonymous' threats -- noting that the website is "wholly separate from any computer network" from the transport infrastructure itself.

An Anonymous press release, issued this weekend, also pointed towards a "massive Black Fax and Email Bomb" in a bid to cause disruption to BART's communications systems; the same disruption seemingly caused to members of the transit system during the protest.

Warning of further attacks, a message on Twitter by an Anonymous account said: "We're not done yet folks... we're just getting warmed up".